1. Field of the Invention
The present invention relates to a developing device including a toner density detecting section in which an output value of detection result changes in accordance with an input control value, an image forming apparatus including the same, and a developing method.
2. Description of the Related Art
An electrophotographic image forming apparatus is widely used, for example, as a copier, a printer, and a facsimile, because a high-quality image can be quickly formed through a simple operation and the maintenance is easily performed. An electrophotographic image forming apparatus includes a photoreceptor, a charging device, an exposure device, a developing device, a transfer device, a fixing device, and a cleaning device. The photoreceptor is a roller-shaped member on which a photosensitive layer is formed. The charging device receives voltage application and charges a surface of the photoreceptor to a predetermined potential. The exposure device applies signal light according to image information to the charged surface of the photoreceptor to form an electrostatic latent image. The developing device supplies toner onto the surface of the photoreceptor to develop the electrostatic latent image into a toner image. The transfer device transfers the toner image on the surface of the photoreceptor to a recording medium. The fixing device fixes, for example, the toner image on the recording medium. An image is thus formed on the recording medium. The cleaning device is a blade-shaped member provided in such a way that the cleaning device abuts on the surface of the photoreceptor, and removes residual toner on the surface of the photoreceptor after the tone image has been transferred to the recording medium.
The developing device includes a developing roller that supplies toner to an electrostatic latent image on the surface of the photoreceptor to form a toner image, a developing tank that stores a two-component developer containing toner and supplies the two-component developer to the developing roller, and a toner density sensor that detects the toner density in the developing tank. The amount of toner to be replenished into the developing tank is controlled in accordance with the detection result obtained from the toner density sensor. The toner density sensor typically outputs a detection result as a voltage, and the output voltage is likely affected, for example, by the detection sensitivity of the toner density sensor itself and the environment in which the two-component developer is used (temperature, humidity, and a cumulative number of printed sheets). For example, the detection sensitivity of the toner density sensor varies with temperature, humidity, and other factors. The detection sensitivity of the toner density sensor also varies, for example, with the speed of printing an image and with number of printed images in the image forming apparatus. In a color image forming apparatus, the detection result from the toner density sensor changes in accordance with a color of toner. Therefore, an adequate amount of toner may not be replenished to the developing tank in some cases, sometimes resulting in reduced image density, faint images, and other problems.
In the image forming apparatus described in Japanese Unexamined Patent Publication JP-A 2005-121951, the input voltage to the toner density sensor is modified, for example, in accordance with temperature, humidity, and the cumulative amount of printed sheets.
In developing devices having toner densities initially set to values similar to one another in consideration of the variation in sensitivity of the toner density sensors, even when printing is performed in similar aging conditions, the toner density control, such as the timing of toner replenishment, may be different among the developing devices. Therefore, after a certain length of time has passed and hence the developer gets degraded, the variation in sensitivity of the toner density sensors has changed and hence differs from the initial value, so that adequate toner densities cannot be obtained. When a plurality of developing devices are provided, for example, in a color image forming apparatus, the sensitivity of the toner density sensor of a developing device differs from the sensitivities of the toner density sensors of the other developing devices, and hence the toner consumption will be different among the developing devices when the whole image forming apparatus is concerned.
In the image forming apparatus described in JP-A 2005-121951, the setting value of the toner density sensor is changed, which, however, does not correct the variation in sensitivity of the toner density sensor. Therefore, an adequate toner density cannot be obtained, and hence the toner consumption will differ among the developing devices.